System for calibration of dual polarization radar with built-in test couplers

ABSTRACT

A calibration system for a dual polarization radar system with built in test couplers has been developed. The system includes a dual polarization radar transmitter antenna that generates a transmission pulse. A test coupler is located behind the antenna that reads a sample of the transmission pulse a test signal. A calibration circuit receives the sample of the transmission pulse and generates a test signal that simulates a desired atmospheric condition. Finally, a test antenna transmits the test signal to the dual polarization radar transmitter antenna for calibration of the system.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a continuation-in-part of U.S. patent applicationSer. No. 12/493,548, now U.S. Pat. No. 8,004,458 entitled “An ImprovedMeans for Dual Polarization Radar with Automatic Built-In Test Equipmentand Calibration” that was filed on Jun. 29, 2009, which is acontinuation application of U.S. patent application Ser. No. 11/952,206,now U.S. Pat. No. 7,554,486 entitled “An Improved System and Method forDual Polarization Radar with Automatic Built-In Test Equipment andCalibration” that was filed on Dec. 7, 2007 and issued on Jun. 30, 2009,which is a continuation-in-part application of U.S. patent applicationSer. No. 11/941,905, now U.S. Pat. No. 7,592,948 entitled “System andMethod for Dual Polarization Radar with Automatic Built-In TestEquipment and Calibration” that was filed on Nov. 16, 2007 which claimspriority from U.S. Provisional Patent Application No. 60/906,730entitled “System and Method for Dual Polarization Radar with AutomaticBuilt-In Test Equipment and Calibration” that was filed on Mar. 13,2007.

FIELD OF THE INVENTION

The present invention relates generally to the field of radar systems.More particularly, the invention provides a system and method for dualpolarization weather radar with built-in test couplers.

BACKGROUND ART

Dual polarization radar systems, also known as polarimetric radar, offeradvantages over conventional radar in many ways. In addition todetecting storms and measuring radial wind velocities, polarimetricradar has been proven by scientists to be the superior radar instrumentfor measurement of rainfall rate (accumulation) and to determine theclassification of hydrometeors, such as wet snow, dry snow, small hail,large hail, graupel, light rain and heavy rain. Many polarimetric radarsystems have been developed and fielded by scientists as instruments tostudy atmospheric sciences, and now some commercial weather radar userssuch as television stations are employing polarimetric radar to moreaccurately measure weather phenomena and to warn the general public ofinclement weather.

Testing and calibration of dual polarization radar instruments have beenmajor difficulties with polarimetric radar. One prior art method forpolarimetric calibration is performed by “bird bathing” the antenna(i.e., directing the antenna straight up into the atmosphere) at a timewhen light-to-medium strataform rainfall covers the radar site. Becauserain drops are almost perfect spheroids, they provide almost equalbackscatter to all radar polarizations (HV/HH=HH/HV). A disadvantage ofthis prior art calibration method is that it can only be performedduring a period of light-to-medium strataform rainfall at the radarsite, and cannot therefore be performed on a regular or as-needed basis.

Another prior art calibration method trains the radar antenna on thesun, which radiates equal amounts of energy in all polarizations. Thismethod is commonly called “sun tracking” or “sun calibration.” With thismethod, HH/HV HV/HH, except that this measurement only measures thereceived signals without regard to the balance of the transmittedsignal. A disadvantage of the sun tracking method of calibration is thatthere are only short periods of time during each day that the sun ispositioned such that an accurate calibration of the receiver can bemade.

It would be desirable to have system and method for calibration of dualpolarization radar that overcomes the disadvantages of the prior artmethods. More specifically, it would be desirable to have method andsystem for calibrating the receiver that can be performed at regularintervals or at any desired time. Another challenge with dualpolarization radar is balancing the power on the horizontal and verticaltransmit channels. It would be desirable to have built-in test equipmentthat adjusts for unequal losses in the horizontal and vertical transmitchannels and provides equal power output to both polarizations.

SUMMARY OF THE INVENTION

In some aspects, the invention relates to a method of calibrating a dualpolarization radar system, comprising: generating a transmission pulsefrom a test antenna for the dual polarization radar system; modifyingthe transmission pulse by capturing a partial wavelength sample of thetransmission pulse with a test coupler located behind the test antennathat is used to generate a test signal that simulates a desiredatmospheric condition; transmitting the test signal directly into theradar system from the test antenna; and calibrating the radar systemaccording to the test signal.

In other aspects, the invention relates to a calibration system for adual polarization radar system, comprising: a dual polarization radartransmitter antenna that generates a transmission pulse; a test couplerlocated behind the antenna that reads a sample of the transmission pulsea test signal; a calibration circuit that receives the sample of thetransmission pulse and generates a test signal that simulates a desiredatmospheric condition; and a test antenna that transmits the test signalto the dual polarization radar transmitter antenna for calibration ofthe system.

Other aspects and advantages of the invention will be apparent from thefollowing description and the appended claims.

BRIEF DESCRIPTION OF DRAWINGS

It should be noted that identical features in different drawings areshown with the same reference numeral.

FIG. 1 is a schematic representation of the components of a systemaccording to one embodiment of the present invention.

FIG. 2 is a graphical plot of the power output when the horizontal andvertical transmit channels are balanced according to one embodiment ofthe present invention.

FIG. 3 is a graphical plot showing the power output when 100% of thepower is applied to the horizontal transmit channel.

FIG. 4 is a block diagram representation of the system with built incalibration components according to one embodiment of the presentinvention.

FIG. 5 is a schematic representation of the Variable Ratio Power Dividercircuit according to one embodiment of the present invention.

FIG. 6 is a schematic representation of the components of a system foran alternative embodiment of the present invention.

DETAILED DESCRIPTION

The present invention provides a system and method for reliable,built-in calibration and testing of dual polarization radar systems. Thepresent invention achieves this object with a unique calibration methodin which both the balance of the transmitted energy and the balance ofthe received energy can be measured precisely and accounted for in theconstants of the radar signal processor. This calibration can be done atregular intervals and at any time of day. The invention provides for theabsolute balance of power of the transmitted signals. The invention alsoprovides for “closed loop” testing of the receiver by simulatingpolarimetric radar signals and injecting the signals into the antenna.Consequently, the present invention enables the radar system to “selfcalibrate” without the use of any external test equipment and in afashion that the radar can function, calibrate and test in the manner ofa remotely located “robot.”

The invention includes a variable ratio power divider that provides theability to completely balance the transmitted power in the horizontally-and vertically-polarized channels by shifting and combining the twophases of microwave signals. Test results, shown in FIG. 2 and FIG. 3,demonstrate the vertically and horizontally transmitted power balanceobtained is better than 1120th of a decibel. Specifically, FIG. 2 is aplot of the power output when the horizontal and vertical transmitchannels are balanced. FIG. 3 is a plot showing the power output when100% of the power is applied to the horizontal transmit channel. Thepower measurement is only limited by the accuracy of the test points andlaboratory test equipment employed in the measurement.

The invention also includes a “snippet circuit” that obtains a shortsnippet or sample (e.g., 30 to 100 meters in width) of the transmittersample pulse to use as a transmitter phase burst pulse, or “lock pulse.”The normal polarimetric radar system transmits pulses ranging in widthfrom approximately 100 meters to several hundreds of meters. A shortsnippet of the transmitter pulse is captured and saved to be used as areference for comparison with the received transmission. Using thesnippet circuit, the system can self-measure the phase droop or phasedelay across the remainder of the transmitted pulse.

FIG. 1 is a schematic representation of one embodiment of a dualpolarization radar according to the present invention. In a coherentradar system, a STALO 81 signal would be sent upstairs (i.e., above therotary coupler, not illustrated) and would be used to convert thesignals to an IF frequency. The transmitted signal would come fromtransmitter 10 through forward coupler 82 to sample port 20 where a veryshort snippet of the transmitted pulse would be “picked off” Forexample, for a transmitted pulse that is 300 meters wide, the user maychoose to snip off the first 30 or 40 meters of the transmission pulse.The “snipped” signal would be transmitted to the IF digitizer 21 to setup a reference for development of Doppler signals.

The remaining pulse would pass through waveguide switch 37 and thenthrough Variable Ratio Power Divider (“VRPD”) circuit 22 en route to theantenna. The VRPD circuit 22 splits the signal two ways in a zero-degreephase relationship. One arm 24 of the VRPD circuit 22 contains a 90degree phase shifter 26. The other arm 25 of the VRPD circuit 22contains a variable phase shifter 27 capable of varying the phase fromzero to 180 degrees.

If the signals in arms 24 and 25 of the VRPD circuit 22 are in phase,they will come out arm 28 on the VPRD and go on to transmit inhorizontal polarization. The waveguide switch 29 can be activated to putan additional 90 degree phase shift (via phase shifter 32) in the signalin arm 24 of the VRPD circuit 22. Then the phase shifter 32 can beadjusted to equally divide the signal measured at ports 30 and 31feeding the H and V ports on the antenna. For example, with a 750kilowatt transmitter, all of the signal could be transmitted through thehorizontal, or 375 kilowatts could be transmitted through each of thehorizontal and vertical, with the phase shifter 32 allowing a precisebalance. Another component of the built-in test equipment is adual-sensor peak power meter 33 that is reading the transmitted powerdown to the third digit, a very accurate level of measurement.

One of the problems in conventional dual polarization radar is that oncethe power is divided, there is no way to balance the power. So if thelosses inherent in components on the horizontal channel (such as 4-portcirculator 34 and couplers 30) are different from the losses incomponents on the vertical channel (such as 4-port circulator 35 andcoupler 31), then unequal power is transmitted. The present inventionachieves the goal of transmitting exactly equal power on the verticaland horizontal channels.

Signals sent from the radar would go out and propagate out in space andwould hit some type of weather event and the radar would receivebackscattered energy from the weather event. The backscattered energywill be captured by the radar signals that are reflected to the dish 51back to the feed 53. The signals travel through individual vertical andhorizontal channels 90 and 91 respectively, and down through receiver 60where they are amplified and then to the IF digitizer 21 where they aredigitized and compared with the snippet that was transmitted to the IFdigitizer earlier.

When comparing the snippet with the received signal, the different phaseshifts in the signal represent velocity. The width of the spectrumrepresents turbulence. The amount of signal in each channel representsthe amount of rainfall or reflectivity. The signal also providesinformation regarding the shape of the raindrop and whether it is frozenor unfrozen, because all that energy is on the reflected signal comingback in.

In the present invention, the signals are simulated by taking thetransmitted signal from transmitter 10 and activating waveguide switch37 to switch from the normal path of transmit (through the VRPD circuit22) over to a dummy load 38 via directional coupler 39. Then, a smallportion of the transmitted signal is directed through leg 40 intocalibration circuit 92. Optionally, this signal path may also employ amicrowave delay line to delay the pulse. For example, if the transmittedsignal is 750 kilowatts in the dummy load, 20 milliwatts (typical value)of this signal could be directed out of the directional coupler 39 intothe calibration circuit 92 through leg 40. In the calibration circuit92, the signal can be modified in phase and amplitude, and can betransmitted from test signal antenna 50 mounted in the vertex of theradar antenna 51.

The test antenna is centered at the vertex of the radar antenna in sucha position that it falls within the shadow of the operational antennafeed, which consists of an Orthomode Transducer or multi-mode(polarization diverse, multiple simultaneous polarizations or variablepolarizations) antenna feed assembly. The test antenna is used toradiate a small low level signal directly to the normal antenna feed.The calibration circuit is mounted on the antenna above the rotarycoupler in a fashion similar to the AN/FPS-16, AN/MPS-T9, M-33, NIKE andnumerous other Military radar systems that have been around for manyyears. In some cases, the stable local oscillator (“STALO”) and/orreference clock are mounted below the rotary coupler.

The invention includes a built in test point to accept the fulltransmitted pulse width and power and an associated calibration circuitthat extracts a portion of the signal and manipulates the signal suchthat the signature in phase and amplitude are representative of what isfound in backscattered energy from a meteorological hydrometeor. Inother words, the invention can simulate microwave backscatter thatexists in weather conditions of interest to the user. The calibrationcircuit then transmits these signals from a test antenna to the mainantenna feed.

The test signal antenna 50 is in the shadow 52 of the feed 53 andtherefore does not affect the overall performance of the radar antenna51 as far as side lobes and distortion of the beam, so it has no effecton the normal radar operations. In the calibration circuit 92, variouscomponents and circuitry are used to modify the signal to take oncharacteristics simulating attributes of various weather phenomena, suchas Z_(DR), PHV, φ_(DP) and K_(DP), where:

Z_(DR)=differential reflectivity;

PHV (Rho_(HV))=H−V correlation coefficient;

φ_(DP) (PHI_(DP))=differential propagation phase; and

φ_(DP)=phase differential with distance or specific phase differentialthat is used to estimate the amount of precipitation in the scannedvolume of atmosphere.

For example, step attenuator 41 can be adjusted to attenuate the signalto simulate rainfall intensity or reflectivity. The resultant signal canbe transmitted through test antenna 50 into the feed 53 and receivedthrough normal circuitry via receiver 60. Then the amount of attenuationreceived could be measured to calibrate the reflectivity of the radar(i.e., to test that the reflectivity range of the radar and accuracy arewithin specification). Attenuator 41 can then be set back to zero.

The calibration circuit 92 also contains a digital phase shifter 42 thatcan be used to impose a Doppler phase shift in the signal. The signalcan then be transmitted from test signal antenna 50 into the feed 53 andback through the normal channels to the receiver 60 and the Dopplervelocity would be read out in the equipment below in the normal radar.

Using digital phase shifter 42, a very noisy sine wave can be imposed onthe signal instead of a pure sine wave, in order to widen the spectrum.Using multiple frequencies mixed together, the Gaussian distribution ofthe waveform can be expanded. The velocity of the spectrum width can besimulated by the modulation of the phase shifter, by the way the signalsare serrodyning.

Another feature of calibration circuit 92 is the digital beam formingcircuit 43, which is very similar to VRPD circuit 22, except that thedigital beam forming circuit is made with coaxial components and useslow microwave power (e.g., 1 milliWatt) instead of high power (e.g., 200Watts). In the digital beam forming circuit 43, the signals coming outof a 90 degree hybrid coupler 44 are in phase, and on one arm 45 of thecircuit 43, a constant length of transmission line provides a fixedphase from coupler 44 to coupler 48. On arm 46 of the circuit 43,instead of having a phase shifter as in the VRPD circuit 22 for the highpower transmission; there is a low power digital phase shifter 47. Usingthe low power digital phase shifter 47, the phases that are recombiningin this part of the digital beam forming circuit 43 can be varied. Ifthe phases are in phase, the signal goes out the “H” port 70. If theyare 90 degrees out of phase, the signal goes out the “V” port 71. Ifthey are 45 degrees out of phase, the signal goes out of both ports. Bychanging the phase in the low power digital phase shifter 47, a phaselag can be imposed on one channel or the other. This phase lag can beused to simulate other characteristics of a received weather eventsignal, such as K_(dp) and Phi_(dp). Therefore, by controlling the phaseshifter 47, different phases and amplitudes can be generated. In fact,the phase shifter 47 can be serrodyned and the phases will actually“roll” the polarization or generate a circular polarization.

The resultant signals are transmitted by the test antenna 50, arereceived by the main antenna feed 53, come back through the normalprocessing to receiver 60, and the radar can be calibrated using thereceived signals. In operation, each different characteristic issimulated and the system is calibrated for that characteristic one at atime, and all of the characteristics can be tested in as little as tenseconds. With the radar doing a volumetric scan, the system can beprogrammed to calibrate the radar fully at the end of the each scan. Fora non-coherent radar system, the transmission signals are generated viathe up-converter option 72, then “pumped” into the same circuit as thecoherent radar, discussed above. As shown in FIG. 1, dotted lines 80illustrate a system used for a non-coherent radar.

The system according to the present invention also includes a digitalnoise source 73 and a power divider 74 that will, through coax switches75 and 76, provide a noise signal into the receiver to check eachchannel and subsequently allow calibration of the transmitter of thesystem. This is an alternative calibration of the receiver similar tothe sun-tracking calibration. Using the system and method according tothe present invention, the transmitted power signal can be completelybalanced during the calibration process. Further, the receivercalibration system and method disclosed herein can simulate “birdbathing” of the radar antenna by amplitude modulation and phasemodulation, by polarization modulation, and imposition of RF phaselead/lag in the beam forming network.

FIG. 4 shows a block diagram of one example of the present invention abuilt in calibration system 100. The calibration system 100 includes aVariable Ratio Power Divider (VPRD) 102 that is similar to the VRPD 22previously described and shown in FIG. 1. FIG. 5 shows a detailedschematic 104 of the VRPD. The VRPD as shown in FIG. 4 and FIG. 6 usesseparate 0-180° adjustable phase shifters 106 a, 106 b and 106 c. For afull transfer of power to the horizontal output 108 a, the VRPD targetsa 0° phase shift of the input 110. For an equal split power splitbetween horizontal and vertical outputs 108 a and 108 b, a 45° phaseshift of the input is targeted by the VRPD. For a full transfer of powerto the horizontal output 108 b, the VRPD targets a 90° phase shiftbetween the horizontal and vertical outputs 108 a and 108 b. For a fullpower transfer to either to either the horizontal or vertical outputs108 a and 108 b (with a 0° or a 90° phase shift), the waveguide switch112 is placed in “position 1” and the signal bypasses a phase shifter106 b. For a power split between the horizontal or vertical outputs 108a and 108 b (with a 45° phase shift), the waveguide switch 112 is placedin “position 2” and the signal engages the phase shifter 106 b. In thisexample, the signal input 110 from the radar transmitter operates in afrequency range of 2.7-2.9 GHz. The input also operates with a 1000 kWpeak pulse and a 1500 W average power levels. The tolerance for powerdivision in 1/20^(th) dB.

FIG. 6 shows a schematic of one example of the system that is used tocalibrate the receiver of the dual polarization radar system. The systemuses two separate methods: attenuation of a test signal; and usingsimulated background noise in conjunction with a test signal. The methodinvolves sampling a test signal 114 to calibrate the receiver 116 of thesystem. The test signal may either be a continuous wave or a pulse. Thetest signal is attenuated or “stepped down” linearly to check both thehorizontal and vertical polarizations. This method can be used tosimulate various weather conditions (e.g., wind speed) with a digital RFattenuator 118 and digital RF phase shifter 120. This simulates theDoppler shift of the desired weather condition in the horizontal andvertical polarities. Once the test signal is adjusted to the desiredconditions, it is transmitted by the test feed 122 to the receiver 116.

The second method of calibrating the receiver involves the use of arecorded level of background atmospheric noise 124. In this example, thebackground noise has a 30 dB ENR (excess noise ratio). The noise is feedto a 3 dB power divider and the signal power is divided evenly betweenbetween the horizontal and vertical changes for a noise level of 27 dBin each channel. This noise level is then feed to the receiver 116 toserve as a calibration signal.

In an alternative embodiment, the present invention would include a testcouplers that was built into the calibration system. The coupler islocated behind the antenna to intercept and inject test signals into thesystem. The test coupler is a directive device that samples the testsignal. It reads the power of the signal going in the forward directionand injects a test signal in the reverse direction to simulate receptionof the signal for calibration purposes. The test sign is typicallydelayed by 10 ms. There is 30 dB isolation between the test coupler andthe antenna. The 30 dB isolation has the advantage of prevention ofcorruption of the test signal reading.

While the invention has been described with respect to a limited numberof embodiments, those skilled in the art, having benefit of thisdisclosure, will appreciate that other embodiments can be devised whichdo not depart from the scope of the invention as disclosed here.Accordingly, the scope of the invention should be limited only by theattached claims.

1. A method of calibrating a dual polarization radar system, comprising:generating a transmission pulse from a test antenna for the dualpolarization radar system; modifying the transmission pulse by capturinga partial wavelength sample of the transmission pulse with a testcoupler located behind the test antenna that is used to generate a testsignal that simulates a desired atmospheric condition; transmitting thetest signal directly into the radar system from the test antenna; andcalibrating the radar system according to the test signal.
 2. The methodof claim 1, where the test coupler is in 30 dB isolation from the testantenna.
 3. The method of claim 1, where the test signal lags thetransmission pulse by 30 ms.
 4. A calibration system for a dualpolarization radar system, comprising: a dual polarization radartransmitter antenna that generates a transmission pulse; a test couplerlocated behind the antenna that reads a sample of the transmission pulsea test signal; a calibration circuit that receives the sample of thetransmission pulse and generates a test signal that simulates a desiredatmospheric condition; and a test antenna that transmits the test signalto the dual polarization radar transmitter antenna for calibration ofthe system.
 5. The system of claim 4, where the test coupler is in 30 dBisolation from the test antenna.
 6. The system of claim 4, where thetest signal lags the transmission pulse by 30 ms.